1. Field of the Invention
An aspect of the present invention relates to a recording medium, and more particularly, to a multilayer recording medium that prevents deterioration of a signal characteristic due to a mirror effect that may occur between recording layers.
2. Description of the Related Art
To increase the storage capacity of a single-layer recording medium, many studies have been performed to develop recording media having a super-resolution near-field structure and multilayer recording media using a short wavelength of a laser beam and a high numerical aperture of an objective lens. As a result of the studies, a Blu-ray disc having a storage capacity of 25 GB per layer in a multi-layer structure has been developed using a blue-violet laser diode and a 0.85 numerical aperture (NA) objective lens. The Blu-ray disc can be used to record about 2 hours of high-definition television (HDTV) or about 13 hours of standard-definition TV.
Storage capacity can be increased using a multilayer recording medium since it is compatible with a single-layer optical disc and capable of being used in a conventional optical pickup. Examples of a multilayer recording medium include a dual-layer digital versatile disc read-only memory (DVD-ROM) disc, a ROM/RAM (rewritable) disc, a write-once (WO) disc, and a RAM/RAM disc. A multilayer recording medium has a capacity N times greater (N is the number of recording layers) than a single-layer recording medium.
In the structure of a multilayer recording medium, the thickness and reflectivity of a spacer layer are very important to the signal characteristics of the multilayer recording medium. The spacer layer should have a thickness of at least 10 μm to prevent crosstalk between recording layers. In addition, the reflectivity of each recording layer should be matched so that a signal detected by a photodetector has at least a predetermined intensity. Moreover, to compensate for spherical aberration, the total thickness of all layers in the multilayer recording medium should be within a compensation range of a compensation apparatus such as a beam expander.
FIG. 1 illustrates an example of a conventional multilayer recording medium having the structure of a quad-layer WO disc, in which a palladium (Pd) oxide layer doped with tellurium (TE), i.e., a Te—O—Pd layer is a layer that data is recorded onto and reproduced from. Referring to FIG. 1, the conventional multilayer recording medium includes a spacer layer having a thickness of 20 μm between adjacent recording layers to decrease the crosstalk therebetween. Spacer layers have the same thickness. In this structure, a mirror effect in which a beam reflected from one recording layer focuses on another recording layer occurs and results in a lot of crosstalk.
FIG. 2 illustrates a conventional multilayer recording medium having recording layers spaced apart a same distance. The mirror effect will be described in detail with reference to FIG. 2 below. Reference characters W1, W2, W3, and W4 denote distances between adjacent recording layers (i.e., thicknesses of respective spacer layers). The distances W1, W2, W3, and W4 are the same. It is assumed that a beam focuses on a recording layer 4 to reproduce data recorded on the recording layer 4. In this case, while the beam focuses at a point “a” on the recording layer 4, the beam reflected from a recording layer 3 focuses at a point “b” on a recording layer 2 and the beam reflected from the recording layer 2 focuses at a point “c” on a recording layer 0 because of the same distance between adjacent recording layers. As a result, due to signals at the points “b” and “c”, a reproducing signal at the point “a” deteriorates, which is referred to as the mirror effect. As described above, the mirror effect occurs when a beam focuses on a first recording layer of a multilayer recording medium to reproduce data from the first recording layer and part of the beam is reflected and focused on a second layer, hereinafter referred to as a mirror layer. For example, in FIG. 2, the recording layer 2 and the recording layer 0 are mirror layers with respect to the recording layer 4.
Since signals are detected from other recording layers besides a target recording layer when reproducing information, the mirror effect deteriorates the quality of a signal and causes a lot of crosstalk. To solve this problem, Japanese Patent Publication No. 2001-155380 has proposed a multilayer recording medium having the structure illustrated in FIGS. 3A and 3B.
Referring to FIG. 3A, a distance between adjacent recording layers monotonously decreases in an advancing direction of an incident beam. Conversely, referring to FIG. 3B, a distance between adjacent recording layers monotonously increases in an advancing direction of an incident beam. In these structures, a beam focusing on one recording layer does not focus on another recording layer and thus the mirror effect is eliminated.
In this case, the mirror effect can be eliminated completely. But the conventional structure of a multilayer recording medium has some drawbacks. First, in these structures, compensation of spherical aberration is not considered. As described above, the total thickness of the multilayer recording medium should be less than a predetermined value. When the multilayer recording medium has a structure in which the thickness of a spacer layer monotonously increases in one direction, the total thickness of the multilayer recording medium may be out of a range for the compensation of spherical aberration. Second, manufacturing the conventional multilayer recording medium with spacer layers having different thicknesses is complicated and costly. Moreover, it is not easy to calculate the reflectivity of each layer in this structure.